If you have spent any time looking through binoculars or telescopes you have undoubtedly come across a double star or two. Someone probably showed you Alberio (beta Cygni) at a star party or tried to impress you with a view of epsilon Lyrae, the famous Double Double in Lyra. One of my favorites is Rigel, the lower foot of Orion. Not many observers know Rigel is actually a double star. It has a 6.8 magnitude companion, Rigel B, 9 arc seconds away. This would be an easy double to separate in most small telescopes, but Rigel is the seventh brightest star in the sky. As such, it is some 400 times brighter than its companion, so Rigel B gets lost in the glare of its primary. Once you know where to look it's easy to find.
Double stars are interesting to people for a number of reasons. Some like the challenge of splitting close pairs with the smallest instrument possible. Others like to measure the characteristics, such as separation, position angle and magnitudes. But what really delights most people is a pair that exhibits a striking color combination. Some of the more popular pairs include Alberio (gold/sapphire), gamma Andromedae (gold/blue), xi Bootis (yellow/red) and alpha Herculis (red/green). I don't want to get into a debate about the perceived colors of these pairs. Your mileage may vary.
My favorite double has them all beat. It is a very colorful pair, with a blue-white primary and a deep red secondary. But the best part is this. It looks different every time you look at it, because the deep red secondary is a variable star! That's right, my favorite double star is also a variable.
You knew that was coming, right?
Okay, okay, I'll end the suspense. My favorite double is the Mira variable T Draconis.
T Draconis resides just north of the head of the dragon
As variable stars go, it couldn't be much better. It's easy to find, located just north of epsilon Dra in the head of the dragon. It varies quite a lot, from 6.7 to 13.2V, and has an excellent sequence. Several of the comparisons from 11th down to 13th magnitude are located very close in to the pair, making it very easy to estimate when its fainter than the blue companion. The next time you find double stars on your observing program for the night, try out T Draconis. Take the time to make an estimate of its brightness and submit it to the AAVSO. Who knows, you just might get hooked. And there are plenty of other interesting double variable stars- TU Aql, T CMi, ST Aur, Z Tau, R Cyg...
The faint stars in the comparison star sequence for T Draconis